1. Field of The Invention
The present invention relates to transceivers, and in particular to microwave frequency-domain radio transceivers using a single antenna with an orthogonal mode transducer and bandpass filtering to separate and isolate transmit and receive signals.
2. State of The Art
A radio transceiver is designed to perform both functions of receiving and transmitting communication signals and as such the transceiver needs to be designed to ensure that the transmit and receive signals do not interfere with each other. FIG. 1 shows a typical transceiver designed to minimize interference between received and transmitted signals. As shown, the transceiver 10 includes a transmit branch, a receive branch, a single antenna for transmitting and receiving orthogonally polarized signals and an orthogonal mode transducer (O.M.T. 11).
The antenna receives signals at a first frequency and transmits signals at a second frequency separated from the first frequency by a predetermined bandwidth. The O.M.T. provides polarization isolation between the two branches, and in particular, a linear polarization O.M.T. functions to maintain a 90 degree spatial difference between the E-fields of the individual receive and transmit guided mode signals in the vicinity of the antenna.
The receive branch includes a bandpass filter (B.P. 12) centered at the frequency of the receive signal such that it passes any signals in this range and rejects signals in the transmit signal range. For instance, if the receive frequency is 28 Giga Hz and the transmit frequency is 30 Giga Hz then B.P. 12 would be tuned to pass 28 Giga Hz signals and reject signals at 30 Giga Hz. It is well known in the field of circuit design that a bandpass filter is designed to allow certain signal frequencies to pass while rejecting other signals at varying degrees of power levels. The rejection level is dependent upon the frequency separation with higher rejection possible as the separation increases. For instance in the case in which the B.P. filter 12 rejects 30 GHz signals by 30 dB, when filtering a 30 GHz signal at 1 m Watt, only a 30 GHz signal at 1 .mu.Watt is leaked to the receiver mixer 13 having an insignificant affect on the mixer 13. The mixer 13 multiplies the high frequency receive signal with a lower frequency signal, LO 14, to generate a received IF output signal that is more suitable for signal processing.
The transmit side also includes a bandpass filter, however, this filter is tuned to pass signals in the frequency range of the transmit signals and reject signals in the frequency range of the receive signals. For instance, B.P. 15 passes 30 Ghz transmit signals and rejects receive signals in the range of 28 Ghz. The purpose of this filter is to reject noise generated by amplifier 16 having frequencies in the receive signal range which may leak over to the receiver branch.
Typically, the input/output (I/O) portion of the above circuit (i.e., antenna 19, O.M.T. 11, B.P.12, and B.P. 15) has been implemented with common discrete waveguiding elements and in particular, in the case of a microwave frequency design, orthogonal mode waveguide coupling structures have been used as well in combination with microwave bandpass filter designs.
The present invention is a low cost unique transceiver I/O OMT-Filter design implementation which effectively and easily provides both isolation and polarization filtering of transmit and receive signals on a PCB-based structure.